DUE GlobAEROSOL: ORAC The Oxford-RAL Aerosol and Cloud retrieval scheme

1. DUE GlobAEROSOL: ORAC The Oxford-RAL Aerosol and Cloud retrieval scheme Gareth Thomas gthomas@atm.ox.ac.uk

2. Introduction • ORAC is the algorithm used to produce aerosol properties from ATSR-2, AATSR and SEVIRI for GlobAEROSOL. • The algorithm has been jointly developed by the University of Oxford and the Rutherford Appleton Laboratory. • This talk gives an outline of the algorithm, including recent improvements which will be incorporated into the GlobAEROSOL processing chain for the production of the full product.

3. People and organisations • University of Oxford • Gareth Thomas • Elisa Carboni • Andy Sayer • Don Grainger • Rutherford Appleton Laboratory • Richard Siddans • Caroline Poulsen • Brian Kerridge

4. ORAC in GlobAEROSOL • ORAC is being used to retrieval aerosol properties from ATSR-2, AATSR and SEVIRI data in GlobAEROSOL: • The (A)ATSR implementation was developed, and is supported by the University of Oxford. • The Rutherford Appleton Laboratory developed and supports the SEVIRI implementation. • The basic algorithm can perform cloud retrievals as well as aerosol retrievals, but this capability is not available in the GlobAEROSOL processing chain.

5. Algorithm overview: Optimal estimation Initial state estimate: x0 A priori: xa Run forward model: f(xi) Compare to J = [y - f(xi)]Sy-1[y - f(xi)] + measurements (y): [xi - xa]Sa-1[xi - xa] Update state: xi→ xi+1(Levenburg-Marquardt) Stop when: J is small , or when i is large.

6. Forward model • ORAC uses an off-line forward model to produce lookup-tables (LUTs) of observed radiance as function of: • Aerosol optical depth • Aerosol effective radius • Viewing geometry • These LUTs are then used as inputs for a simple analytical forward model expression (the Fast-FM) within the retrieval scheme itself.

7. Off-line forward model:Aerosol properties Polluted Biomass burning 10.0 Maritime: - water soluble - sea salt (acc.) - sea salt (coa.) Continental: - water soluble - insoluble Desert: - water soluble - mineral (nuc.) - mineral (acc.) - mineral (coa.) Polluted: - soot - water soluble - insoluble 1.0 Effective Radius [mm] 0.1 550nm Ch1 Ch2 Ch3 0.01 550nm Ch1 Ch2 Ch3 Ksca(l)/Kext(550nm) Scattering coefficient normalized to 550nm Maritime Continental Desert 550nm Ch1 Ch2 Ch3 550nm Ch1 Ch2 Ch3 550nm Ch1 Ch2 Ch3 Aerosol components from OPAC database [Hess et al. 1998] • Biomass Burning • (Cerrado): • fine mode • - coarse mode water soluble and sea salt component RH 50% mineral, soot and insoluble components are considered non hygroscopic [Dubovik et al. 2002] SEVIRI

8. Off-line forward model:Aerosol scattering and extinction Microphysical properties Every component is characterized by: Spectral refractive index r(l) + i m(l) Mode radius rm and spread s Number density described by a log normal size distribution Changing the mixing ratio between component we obtain the optical properties corresponding to different effective radii Kext() () < 1 P(, )

9. Offline forward model:Lookup-table generation Aerosol Spectral optical properties Kext() () < 1 Look Up Tables Mie theory (spherical approx.) Instrument’s filter characterization Radiative transfer model DISORT P(, ) Molecules gas(,h) Molecular scattering Molecules + aerosol m() Pm(, ) Molecules Gas absorption profile MODTRAN computations Aerosol Microphysical properties Plane parallel Atmosphere tatm(,h) atm(,h) Refractive index r() + i m() Patm(,,h) Size distributions N(r) Mixing ratio Black surface

10. Surface reflectance • An accurate description of the surface reflectance is of vital important to the ORAC aerosol retrieval. • ORAC retrieves surface reflectance, but an accurate apriori estimate is still important. • How the a priori/first guess surface reflectance is set depends on the type of surface: • Over the ocean, a reflectance model, based on the method of Koepke (1984) is used. • Over the land, the MODIS BRDF product (MOD43B) is used. P. Koepke. Appl. Opt., 23:1816–1824, 1984

11. Fast-Forward Models • There exist two Fast-FMs for ORAC aerosol retrieval: • One assumes the surface can be treated as a Lambertian reflector. Used in the production of the month long (A)ATSR PPS available from the GlobAEROSOL website • One uses a Bidirectional Reflectance Distribution Function description of the surface reflectance. This has been recently developed: Used in the generation of the SEVIRI PPS Although not yet fully integrated into the GlobAEROSOL (A)ATSR processor, products are have been incorporated into the PPS

12. Lambertian Fast-FM

13. Surface reflectance retrieval As well as the aerosol optical depth and effective radius tabulated in the LUTs, the Lambertian ORAC also retrieves an effective surface reflectance at 550 nm. • The reflectance is based on the Lambertian approximation, but depends on viewing geometry • The spectral shape of the surface reflectance is fixed by a priori constraint. Retrieved True Reflectance A priori λ

14. BRDF Fast-FM

15. Why a new Fast-FM? The use of a BRDF description of the surface reflectance offers important advantages: • It provides a more accurate description of the contribution of the surface to the measured signal, especially for certain land surfaces and sun-glint effected water. • In the case of (A)ATSR, the surface reflectance at the two viewing angles can be defined in a consistent manner, allowing both views to be incorporated into the retrieval.

16. GlobAEROSOL implementation:Extra products • Since GlobAEROSOL requires optical depth at two wavelengths (550 and 870 nm) and the Ångström coefficient, a modified version of the retrieval is used. • A LUT of the ratio of the extinction coefficient (βext) between 550 and 870 nm as a function of optical depth and effective radius has been included in the retrieval. • Optical depth at 870 nm and the Ångström coefficient are then calculated from the 550 nm optical depth and this ratio

17. GlobAEROSOL implementation:Variation between instruments • AATSR retrieval • Makes use of channels 1-4 (0.55, 0.67, 0.87 and 1.6 μm) • Cloud flagging over the ocean uses the ESA operational flag, while over the land an in-house scheme based on the NDVI is used • ATSR-2 retrieval • Does not include the 0.55 μm channel, as this is often in “narrow swath mode” • Utilises an in-house thermal-IR and NDVI cloud flag over the sea • SEVIRI retrieval • Makes use of channels 1-3 (0.67, 0.87 and 1.6 μm) • Uses the EUMetSat operational cloud mask.

18. Dual view (A)ATSR retrieval • The inclusion of both views in the (A)ATSR retrieval is a major improvement in the algorithm: • Under the assumption that the aerosol properties along the two viewing directions are the same, the two views make it much easier to distinguish the surface and atmospheric contributions to the TOA signal. • The effective doubling of the number measurements available to the retrieval makes it possible to independently retrieve the surface reflectance in all channels.

19. AATSR dual-view vs single viewComparison with AERONET Best fit: y = 0.42x + 0.14 Correlation: 0.26 Best fit: y = 1.18x + 0.01 Correlation: 0.96

20. AATSR dual-view vs single view Lambertian BRDF dual-view 550 nm Optical Depth 870 nm Optical Depth

21. Comparison with MODIS and MISR

22. AATSR dual-view vs single view Lambertian BRDF dual-view Ångström Coefficient Dominant Aerosol Class

23. Po Valley example 20 August 2004

24. AATSR single view, Lambertian surface 20 August 2004 Optical depth at 550 nm

25. AATSR dual-view, BRDF surface 20 August 2004 Optical depth at 550 nm

26. AATSR single view, Lambertian surface 20 August 2004 Error in optical depth at 550 nm

27. AATSR dual-view, BRDF surface 20 August 2004 Error in optical depth at 550 nm

28. Further information on ORAC Lambertian ORAC ATBD for GlobAEROSOL: http://www.globaerosol.info/docs/aatsr_atbd_v1p0.pdf Description of the BRDF fast-forward model: http://www.atm.ox.ac.uk/main/research/technical/2007.3.pdf Description of the ocean surface reflectance model used in ORAC: http://www.atm.ox.ac.uk/main/research/technical/2007.2.pdf Preliminary GlobAEROSOL validation report: http://www.globaerosol.info/docs/globaer_pvar_v1p2.pdf

30. AATSR dual-view vs single view 550 nm optical depth, 13 October 2005 Single view, Lambertian Dual view, BRDF

31. AATSR dual-view vs single view 550 nm optical depth, 13 October 2005 MERIS BAER algorithm MODIS collection 4 After Kokhanovsky et al., Atmos. Res., 85:372-394, 2007